Our recently discovered in vivo method for generating hapten-specific cytotoxic T cells (CTLs) to hapten-altered self-antigens has now permitted examination of the regulation of this response. The CTLs appear in mice given H-2 compatible, minor histoincompatible auxiliary cells along with hapten-modified syngeneic stimulator cells. The disparate minor locus antigens (mls) on the auxiliary cells apparently stimulate the host animal's helper cells to release a helper factor required by the pre-killer T cells. The latter become fully differentiated CTLs oriented to the hapten-modified major histocompatibility self antigens. There are at least three levels of control over the in vivo cell mediated lympholytic system thus far described. One is the naturally existing suppressor cell that prevents development of CTLs toward modified syngeneic antigens. A second is the host's helper cells which, upon stimulation by the proper antigens, can override the natural suppression. The third is the host's precursor CTLs which must perceive the hapten-modified self antigens as foreign. By using immunotolerance methods directed toward Mls antigens as well as toward hapten we have begun to show control exists at the latter two levels. We will examine the events that enhance as well as suppress in vivo development of CTLs. Delayed type hypersensitivity (DTH) will be examined in parallel since it is the other cell mediated immune response towards which tolerance techniques have been directed. Both cell mediated responses as well as tolerance will be directed toward hapten-altered syngeneic as well as allogeneic major and minor histocompatibility antigens. Furthermore, in some experiments we shall be using dextran as an immunoprobe since we have shown that it markedly augments DTH and that it circumvents tolerance induction for the unrelated hapten. In this way we will identify functional control points in the development of CTLs operative in the syngeneic lympholytic system and of effector T cells in DTH. Such knowledge will be useful in the areas of immunological tolerance, autoimmunity and graft as well as tumor rejection.